1. Field of the Invention
The present invention relates to a base oil for lubricating oil, a lubricating oil composition containing the base oil, and an additive for lubricating oil. More particularly, it is concerned with a base oil which is used as a lubricating oil stable in a nitrogen oxide (NO.sub.x) gas atmosphere by itself, or is used to prepare such a stable lubricating oil in combination with suitable additives, a lubricating oil composition containing the above base oil, and further with an additive for the general base oil for lubricating oil.
The term "lubricating oil" as used herein means a lubricating oil for use in internal combustion engines.
2. Description of the Prior Art
In general, part of a combustion gas in internal combustion engines passes through between a piston and a cylinder, and leaks out into a crank case a a blow-by gas. Since this combustion gas contains a high concentration of NO.sub.x gas, it deteriorates a crank case oil (internal combustion engine oil).
In recent years, cars equipped with a reduction catalyst such as a three-way conversion catalyst as a countermeasure of exhausted gas regulations have been increasingly produced, and thus internal combustion engines are now operated under more severe conditions of high speed and high power. As a result, the concentration of NO.sub.x gas in combustion gas leaking out into the crank case tends to increase.
Moreover, from a viewpoint of energy saving, it is promoted to make the car body lighter, and thus the crank case is miniaturized. With this miniaturization, the amount of the crank case oil is decreased.
For the aforementioned reasons, the concentration of NO.sub.x gas in the crank case oil is markedly increased, and thus the crank case oil is greatly influenced by NO.sub.x gas. With the conventional internal combustion engine oils containing zinc dithiophosphate (Zn-DTP) and a detergent dispersant, abnormal degradation such as the formation of black sludge will occur in a short time.
Thus it has been desired to overcome the above problems and to provide a base oil or lubricating oil which is stable in a NO.sub.x gas atmosphere.